One of the most important and sensitive aspects of well drilling operations involves controlling the rate of fluid transfer between the well and the various subterranean rock formations surrounding the borehole. Control of this fluid transfer is achieved by varying the properties of the fluid, termed "drilling fluid" or "drilling mud", which is circulated through the well in the course of drilling operations. The drilling fluid serves several purposes in addition to its use in controlling fluid transfer between the rock formations and the borehole, including: cooling and lubricating the drill bit, carrying rock cuttings away from the bottom of the borehole, and supporting the walls of the borehole. Typically, the drilling fluid is injected into the bottom portion of the borehole through the tubular drill string used to drill the well. The drilling fluid returns to surface through the annular portion of the borehole external to the drill string.
As the drill bit penetrates a subterranean formation, that formation is brought into fluid communication with the surface via the borehole. If the pressure of a permeable formation traversed by the borehole exceeds that of the borehole by a sufficient amount, the fluids in the formation (typically water, oil or hydrocarbon gas) can be forced into the borehole under pressure and released to the surface in an uncontrolled manner. This condition is commonly known as a blowout. To prevent blowouts, the density of the drilling fluid is carefully controlled to maintain the pressure in the borehole at a level such that the fluids in permeable formations are prevented from entering the borehole.
Well control problems can also arise if the pressure in the borehole significantly exceeds that of one or more of the formations traversed by the borehole. Should the density of the drilling fluid be greater than that of a permeable formation, it is possible for drilling fluid to be forced into the formation. This condition is termed "lost returns". In some instances the hydrostatic pressure of the drilling fluid can be great enough to fracture a weak formation, causing drilling fluid to pass into the formation at a rapid rate. As an additional complication, should there also be a relatively high pressure formation at another point along the borehole, this loss of drilling fluid to the weak formation can cause a temporary drop in the hydrostatic pressure head of the borehole of sufficient magnitude to induce a blowout from the high pressure formation. To minimize the potential for lost returns, it is usually necessary to control the density of the drilling fluid so that the pressure in the borehole does not greatly exceed that of the weak formations and permeable formations.
The most effective manner of guarding against blowouts is to monitor the well to determine the onset of formation fluid intrusion. If this initial intrusion, commonly referred to as a "kick", is detected at its inception, it is usually not difficult to prevent the situation from advancing to a blowout. Similarly, lost circulation is most easily corrected when the loss of drilling fluid is detected at an early stage.
One of the most common techniques for detecting kicks and lost circulation in the course of drilling operations is delta flow monitoring. Delta flow monitoring involves comparing the rate at which drilling fluid is injected into the well to the rate at which drilling fluid exits the well. After monitoring these rates over a sufficient period of time, it becomes possible to determine the differential ("delta") flow rate. The delta flow rate represents the cumulative change in the amount of drilling fluid within the well over the selected time period. A net addition of drilling fluid to the borehole is indicative of lost returns. Likewise, an excess of returned drilling fluid over injected drilling fluid signals the intrusion of formation fluids, possibly the onset of a blowout. Upon receipt of an indication of such well control problems, remedial measures must be initiated. These remedial measures are usually designed to lessen the pressure differential between the borehole and the surrounding formations, or to seal the permeable formations through which fluid migration is occurring.
Delta flow monitoring poses special difficulties in offshore drilling operations conducted from a floating drilling platform, such as a drillship. Floating drilling operations must accommodate wave-induced motion of the drilling rig relative to the borehole. To accommodate this motion, the marine riser, which serves to extend the borehole from the seafloor to the drillship, is provided with a telescoping slip joint. As vessel motion causes the slip joint to expand and contract, the fluid capacity of the return flowpath for the drilling fluid changes. This introduces nonuniform, cyclical variations in the rate of drilling fluid outflow. These variations mask the true delta flow.
Numerous attempts have been made to develop techniques and apparatus for mitigating or eliminating the effects of vessel heave on delta flow monitoring. One class of such developments involves placing the drilling fluid return flowmeter below rather than above the slip joint. One such system is disclosed in U.S. Pat. No. 3,811,322, issued May 21, 1974. While such systems avoid the effects of vessel heave, they are disadvantageous in that they require that a flow measurement be made of the drilling fluid passing through the annulus defined by a rotating drill string and the non-rotating riser. Obtaining accurate flow measurements over all flow conditions with this arrangement presents numerous mechanical problems. Further, positioning the drilling fluid return flowmeter beneath the slip joint places it in a relatively inaccessible location, rendering repair or replacement difficult should the meter fail.
A second system for eliminating the effects of vessel heave on delta flow measurements involves correcting the rate at which drilling fluid passes through the output flowmeter so that it does not detect the instantaneous component of fluid flow resulting from vessel heave. U.S. Pat. No. 4,135,841, issued Jan. 23, 1979, discloses a heave compensator which causes a change in the fluid volume of the drilling fluid return line equal and opposite to the change caused by motion of the slip joint. Use of this heave compensator substantially eliminates the effect of vessel heave on the flow rate, but is disadvantageous in that it requires complicated and bulky mechanical equipment.
A third system for eliminating the effects of vessel heave involves processing the output signal of the drilling fluid return flowmeter to mitigate the cyclical contribution of slip joint volume change. In one such system, taught in U.S. Pat. No. 4,282,939, issued Aug. 11, 1981, the delta flow measurement is averaged over a time period extending from when the slip joint is in a given reference position until the vessel goes through one complete heave cycle, returning to that reference position. In this manner, flow rate fluctuations due to vessel heave are substantially eliminated. A disadvantage of this system is that a sensor is required on the slip joint. This complicates installation and maintenance of the delta flow system. Further, averaging the delta flow over only a single vessel heave cycle yields an averaging period too short to adequately diminish cyclical variations in delta flow resulting from causes other than vessel heave, such as vessel pitch and yaw. Another such system, disclosed in U.S. Pat. No. 4,440,239, issued Apr. 3, 1984, utilizes an output signal filter having a long and a short time constant selectively applied in response to the magnitude of vessel heave. This system is disadvantageous in that in many instances the applied filtering is too severe, causing excessive lag in the response of the delta flow monitoring system. Excessive filtering desensitizes the monitoring system, introducing an unduly great lag time between the onset of the well fluid control problem and detection of the problem. Conversely, too little filtering can result in false alarms, resulting from wave induced flowrate oscillations being incorrectly detected as a well control problem.